A bright idea

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Kleinknecht’s EDT machines texturize rollers used in sheet metal processing.

Ironless linear motors move 60 electrodes with great precision.

March 2014

 

Machine tool specialists at Kleinknecht use precise linear motors from Rexroth to build machines for Electronic Discharge Texturing, which texture rollers that are used to scarify the surface of sheet metal. This provides better oil film adhesion during pressing, and painting the metal becomes more economical and environment-friendly.

 
 

Curvy car bodies with perfect paintwork and shiny white home appliances demand sheet metal with a carefully created surface that has been scarified to depths of just microns. This is done with a roller which – applying high pressure – embosses its structure on the surface of the metal, just like minting a coin. Not only do paints and powder coatings adhere better to such surfaces, their application is also significantly more economical. Car manufacturers can save some four kilograms of paint per vehicle, for example.

But how is the roller itself textured? The most accepted method is Electrical Discharge Texturing (EDT). H. Kleinknecht & Co. GmbH, the automation and drive specialists in Siegen, Germany, are following a new path, using linear motors and control technology from Rexroth.

Operating in the sub-micron range

Kleinknecht is the first manufacturer of high-end EDT machines who utilizes electric linear motors to correctly set the electrodes for the electric discharge process. “When developing our EDT machines the challenge was to coordinate the movements of all 60 electrodes and to position them in the sub-micron range. Having achieved this, we are able to create roller surfaces with perfect quality,” is how Christoph Hauck, general manager at Kleinknecht, explains the process. The drive system adjusts the distance between the electrodes and the roller with a precision of under one micron – ten times more accurately than ever before. The system makes use of Ironless IndraDyn L linear motors from the MCL series as well as of compact IndraDrive HCS01 drives.

Kleinknecht’s automation experts programmed the core functionalities for the EDT process directly into the drives’ integrated IndraMotion MLD motion control. An IndraMotion machine controller, communicating with the drives through the Sercos® automation bus, controls the complete cycle from insertion to removal of the roller. Additionally, it links the machine to the customer’s production system by using, Profinet, Profibus, Ethernet/IP, CANopen or a Sercos® network, just as required. Since the Electric Discharge Texturing core functions for each electrode are controlled on decentralized MLDs, a potential bottleneck in the central controller is avoided, thus the process is controlled more reliably, and the system operates more accurately.

Intelligent process control

At the system’s heart are Rexroth’s Ironless MCL linear motors. Their lightweight primary parts with electromagnetic coils move between two rows of permanent magnets in the secondary part. In Kleinknecht’s EDT machines the electrodes are attached to the motor’s primary parts – directly, and mechanically very rigid. The high servo gain, that is possible with this direct drive setup, enables highly dynamic movements and very accurate positioning in the shortest possible time. Furthermore, the control behavior of Rexroth’s linear motors is constant across the entire travel distance. These characteristics ensure that the required electrode clearances are maintained precisely. Over and above this, MCL linear motors produce no cogging forces; this means less disturbance is carried into the servo positioning loop.

The low dead weight and small inductivity of these Ironless drives also make for high acceleration and high travel speed. Thus even vibrations caused by some incident in the vicinity of the EDT machine can be compensated, otherwise potentially affecting the roller’s surface quality. “The results of the very first test runs already made clear that Kleinknecht’s innovation has great potential,” says Jan Schönerstedt, product manager for Direct Drives at Bosch Rexroth. “Textured surfaces can be created under much closer control than is the case with conventional EDT machines. Furthermore, the process became considerably more reliable.” Kleinknecht’s new approach has proved its worth on many occasions under practical conditions.

Using Rexroth drive and control components throughout does simplify planning, commissioning and maintenance of Kleinknecht’s EDT machines. In addition, new possibilities to create roller surfaces can be offered: Since all drive parameters and process characteristics are accessible in real time, operators of Kleinknecht EDT machines develop new approaches to Electric Discharge Texturing.

An open software structure

On the software side Kleinknecht’s customers benefit from the open structure of Rexroth’s solutions. The IndraWorks suite combines software for programming, configuring and commissioning in a single, clearly arranged user interface. The integration of project administration for central data management of the machine’s configuration, a visualization framework, and the PLC programming ensures the consistency of the data.

With the PLCopen automation architecture, machine operators can easily integrate the software for EDT machines into their production systems. Intuitive help menus are available to support users with every step – from configuring the machine to parameter setting for technical functions. Christoph Hauck: “A significant benefit of the open structure is that it requires no proprietary field buses or programming environments. This means that EDT machine software can easily be adapted to the customer’s changing needs at any time – adding confidence when making long-term investments.”

 

Electric Discharge Texturing

The roller (1) is rotated in a dielectric fluid (2) and simultaneously oscillated in its axial direction (3). Up to 60 electrodes (4) are moved radially towards the surface of the roller until they are mere microns away from it. During the texturing process, a large number of microscopic electrical discharges is emitted at precisely controlled intervals and currents. Microscopic sparks (5) erode the roller’s surface to give it the desired texture. Accurately maintaining the distances between the electrodes and the roller’s surface is decisive for the surface quality. Shaft runout and electrode burn must be continually compensated for.